The present invention relates a branch circuit monitor and, more particularly, to a branch circuit monitor having memory including a paging register.
The total power consumption of a building or other facility is typically monitored by the electric utility with a power meter located between the utility's distribution transformer and the facility's power distribution panel. However, in many instances it is desirable to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility or to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, etc. These single phase or multi-phase electrical loads are typically connected to one or more of the branch circuits that extend from the facility's power distribution panel.
While a power meter may be installed at any location between a load and the distribution panel, typically a power meter capable of monitoring a plurality of circuits is installed proximate the power distribution panel to provide centralized monitoring of the various loads powered from the panel. For example, while the voltage transducer(s) of a power meter can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals, the power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is conducted by a separate bus bar in the power distribution panel and the voltage and phase is the same for all loads attached to the respective bus bar. The voltage transducers of digital power meters commonly comprise voltage divider networks that are connected to a bus bar or other conductor in which the voltage will be measured.
The current transducers typically comprise current transformers that encircle the respective power cables that interconnect each branch circuit to the bus bar(s) of the distribution panel. A current transformer typically comprises multiple turns of wire wrapped around the cross-section of a toroidal core. The power cable, conducting the current to be measured, is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer. The wire wrapped around the cross-section of the core comprises the secondary winding of the transformer. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is quantitatively related to the current in the primary winding. The secondary winding is typically connected to a resistor network and the magnitude of the primary current can be determined from the amplitude of the voltage at the output of the resistor network.
Flexibility has favored adoption of digital current and power meters, known as branch circuit monitors, incorporating data processing systems typically comprising one or more microprocessors or digital signal processors (DSP) that periodically read the output of each of the voltage and current transducers, calculate the current or voltage at the respective transducer and display or store the results. In addition, the data processing unit periodically may calculate the power and other electrical parameters, such as active power, apparent power and reactive power that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing and/or transmitted from a communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.
Branch circuit monitors are commonly incorporated in networks that utilize the MODBUS® serial communication protocol. The MODBUS protocol, first published in 1979, enables serial communication between up to 247 interconnected network devices and has become a de facto standard communications protocol for connecting industrial electronic devices. Each device is given a unique address and each query contains the address of the intended device. A device will not respond to a query unless it contains that device's address. A basic MODBUS query can control or read an input/output port of a receiving device or instruct the receiving device to change or send back one or more values contained in its memory.
While the MODBUS protocol is a de facto standard for industrial electronic devices, it does have its limitations. For example, since MODBUS was developed in the 1970s to communicate with programmable logic controllers (PLCs), the number of supported data types is limited to those utilized by PLCs at that time and does not include large binary objects. Also, the number of addressable registers in a MODBUS device is limited which can affect the device's performance and utility. For example, a branch circuit monitor monitoring a large number of circuits and determining several electrical measurement parameters for each circuit generates substantial amounts of data and the limited number of addressable registers can limit the number of circuits monitored by the branch circuit monitor and/or the types of data that can be produced and communicated by the monitor. What is desired, therefore, is a MODBUS network connectable branch circuit monitor with increased addressable memory.